Numerically controlled machine tools are widely used for machining many types of parts. In the aircraft industry, gantry-type and post mill-type machines having multi-axis movement capabilities are used for machining wing and fuselage panels to form holes in which rivets, bolts, or similar fasteners will be installed for attaching various structures and components to the panels. The panels in many cases are quite large, and are held for machining by a flexible workpiece holding fixture whose configuration can be varied to match a given workpiece so that panels of various configurations can be fixtured. A machine tool, such as a five-axis tool that is translatable along three mutually orthogonal axes and rotatable about two orthogonal axes, is positioned opposite the holding fixture and is translated and rotated to position a tool held in a spindle of the machine in the proper locations for drilling holes through the workpiece or performing other machining operations.
In such a system, it will be appreciated that there are many degrees of freedom between the machine tool and the holding fixture. Accurate parts can be produced only if there is a high degree of confidence that the accuracy in positioning of the machine tool and the holding fixture are within acceptable limits. However, there are many potential sources of errors that can creep in, both within the machine tool and within the holding fixture. Potential sources of errors in the machine include mechanical misalignments of and between the linear ways of the machine along which the machine travels, and mechanical misalignments of and between the rotational axes of the machine. Additionally, where the workpiece holding fixture includes holding elements that can be variably positioned along one or more axes, the fixture may introduce further inaccuracies along and between such axes.
Traditional methods for checking and correcting positioning errors in such machine systems have relied heavily upon recalibrating and realigning the machine to original factory specifications when the errors become unacceptably large. This can involve the replacement of parts of the machine that can no longer deliver performance up to par with the original specifications. Many times, the errors in positioning are judged by inspecting the finished part quality and noting when the parts become out of tolerance. This is an inherently reactive rather than proactive process, and inevitably unacceptable parts will be produced at some point when the machine accuracy declines as a result of wear, shifts between parts of the machine, or other causes.
A further drawback to this traditional approach is that it may well be possible to produce parts within acceptable tolerances even though the machine does not meet original factory specifications. Accordingly, realigning the machine to original specifications may result in needless downtime and expense. In order to efficiently correct inaccuracies without rebuilding the machine to original specifications, however, the root causes of the errors must be known. In the traditional approach to machine accuracy qualification, errors are first noted by checking the finished part quality, but this yields little or no insight into what is causing the parts to be produced out of tolerance. The traditional approach therefore involves a long and cumbersome process of measuring the linearity and straightness of each axis of the machine, the squareness between each pair of orthogonal axes, the alignment of the rotation axes, and other parameters, and correcting any unacceptably large inaccuracy by realigning the axes and replacing parts as needed in order to reestablish the original factory specifications. In this process, the true root causes of errors may not ever be discovered; it is simply hoped that by realigning the machine to original specifications, the finished part quality will be restored to an acceptable level.
A further drawback to many prior machine accuracy qualification methods is that the ultimate error in production part accuracy is never linked mathematically to the various contributing factors in the machine and/or holding fixture for the part, and hence there is no systematic way to check the machine and fixture accuracy that will assure that parts will be produced within acceptable tolerances. Accordingly, it is generally necessary to inspect the finished parts to determine whether the machine system is performing acceptably. It would be desirable to provide a machine accuracy qualification method in which the production part accuracy is mathematically linked to the various potential sources of error in the machine and holding fixture, enabling a high degree of confidence in production part accuracy to be achieved without having to regularly inspect the parts. In short, many prior attempts to maintain a high level of confidence in the accuracy of machines and fixtures have failed because of a misunderstanding of what needs to be checked, because the time between accuracy checks was unacceptably long, and because methods for collecting, analyzing, and reporting measurement data were incomplete and the results difficult to interpret.